Robots are increasingly being employed in tasks that are otherwise dangerous or tedious to humans. The utility of a robot can be increased when tactile sensors are incorporated into the hands or grippers of the robot to enable the robot to "feel" objects in its environment. Ideally, a robotic hand should be able to hold an object tight enough to keep the object from slipping from its grasp, yet be gentle enough to keep from crushing or breaking the object. This desired sensitivity has been typically attempted by mounting a two dimensional array of independent pressure sensitive sensors on the hand of the robot which are linked together by computer software to determine the forces exerted by the object being grasped at different positions on the array. Such arrays can detect only one degree of freedom, i.e., they can only detect forces normal to the robotic hand and cannot sense tangential or torsional forces.
The one-dimensional pressure sensitive arrays of the prior art use a number of different types of sensing elements. The sensing elements may include conductive rubber, piezoresistive and piezoelectric elements, optical methods using indices of refraction, electro-optical transduction, and fiber optics. Each of these methods has its respective advantages and disadvantages, though all suffer from the inability to detect more than just normal force components. The prior art tactile sensors therefore are unable to detect tangential forces created when an object is slipping from the robotic hand, nor can the prior art sensors relay information relating to torsional forces created when an object is grasped away from the center of gravity. Many of the prior art devices have an insufficient range of force detection and high hysteresis, or require long computer processing times.
Although tactile sensors having the ability to detect multiple force components have been the subject of recent research, sensors with such capabilities are presently available only in applications where each force component is measured independently. These sensors are too large for reasonable use in a robotic hand.
For the aforementioned reasons, not many robots are currently using tactile sensors. There are, however, many possible applications for tactile sensors. Among them are adaptive grasping; automated wiring; cutting; handling soft, light, or limp materials; pressure distributions in gaite (feet) as well as whole body analysis; prosthetic systems; machine loading and unloading; detection of jamming during insertion; bulk part removal; rivet insertion; leather grading; weld tracking and inspection; underwater geological prospecting; sheep shearing; industrial floor sweeping; automatic repair (as in space and nuclear reactor installations); chemical and pharmaceutical handling; harvesting; shelling; flower and plant handling; poultry plucking; and cheese ripeness testing. Although not a complete list, this sampling shows that satisfactory sensors could find a wide variety of applications.
To meet these diverse applications, tactile sensors should be three-dimensional and have multiple degrees of freedom, i.e., be able to accurately detect multiple force components. Preferably, the tactile sensor should be able to detect four degrees of freedom one normal force, two tangential forces, and one torsional force or torque. A tactile sensor should be chemically inert and robust, that is, not be sensitive to abuse. The tactile sensor should have minimal manufacturing costs and should preferably be disposable, easily replaced, and/or repairable. A tactile sensor should also be compliant so that it will not damage the object being manipulated and so that the force control of the robotic system can be facilitated. Stiff force sensors are generally undesirable for force controlled robotic systems.